If someone wanted to jack you, they would jack you. L brackets would probably help buy some time but they can just carry some tools around if they were serious about the job. Just don't bump your music near your house or where you're about to park. I have a viper alarm on my car so if someone breaks in, it will alarm my 2 way pager, and I just come out with a bat.
In regards to securing your box in place while driving, velcro works great.